The present invention relates to an electrical snap-action switch comprising a snapping spring made of sheet metal divided into at least two strips extending in parallel with one another, with these strips being joined to one another at both ends by cross webs, and of which at least one strip is unshortened, and at least one other strip is shortened by means of a bending, with the sheet metal carrying at least one electric contact being designed to cooperate with an opposite contact arranged on a flat base, and in which there is provided an actuating device by which the unshortened strip can be caused to snap over.
Electrical snap-action switches have already become known, in which as the snapping element there is used a spring plate which is divided by slots into several strips extending in parallel with one another, and which are joined to one another by cross webs at both ends. If, in the case of such a spring plate, at least one strip is shortened by way of bending or arching, and when at least one other strip is unshortened, the unshortened strip is arched towards one side, thus causing the spring plate to become distorted. With the aid of a suitable actuator, it is now possible to exert a pressure upon the unshortened strip causing the latter to snap over to the other side of the plate's plane. This snap over process may be utilized for closing or opening electrical contacts, with a switch being obtained in this way, whose contacts can be rapidly closed and opened, i.e. independently of the actuation by the actuator. In this way there is not only always established a clearly defined switching condition, but also the sparking between the contacts to be switched is extensively suppressed. Such a type of snap-action switch has become known, for example, from the German Pat. No. 458,899. The flat spring that snaps the contacts in this conventional type of switch consists of three sheet metal strips which are joined to one another and their ends, with the two outer strips each being shortened by two archings formed therein, while the center strip remains unshortened and can be caused to snap over.
Another snap-action switch of this type has become known from the German Pat. No. 912,597, in which there is likewise used a flat snapping spring of three strips connected to one another, of which the two outer strips are each shortened by means of bendings.
From the German Pat. No. 1,806,807 there is known a further electrical snap-action switch in which the flat snapping spring is obtained in that two U-shaped recesses are punched out, and by folding the sheet metal and hooking the individual parts into one another. In this case, the distortion is not only obtained by way of deforming the individual parts of the sheet metal spring, but also by hooking the bent-off sheet metal members into one another.
From the German Published Patent Application (DT-OS) No. 2,356,024 there is known a keyboard in which the individual snap-action switches are provided with flat snapping springs which are clamped at one end, and in which the flat snapping spring as divided into strips, is distorted by providing for shortenings of the outer strips.
In the push button switch according to the German Published Patent Application (DT-OS) No. 2,411,426 there is likewise used a flat snapping spring divided by slots into three coherent strips, with the two outer strips thereof being shortened by means of bendings. The unshortened strip in the center is provided in this case with L-shaped extensions carrying the contacts.
Finally, applicant's earlier German Patent Application No. P 25 37 905 covers an electrical snap-action switch comprising a snapping spring which is divided into strips, with contacts being provided for in the snapping spring on at least one strip, and the snapping spring being extended in the direction of the strips, and beyond the connecting web of the strips in such a way as to form extensions with contacting points, and the snapping spring, at least within the area of the connecting web extended by the extension, being mounted capable of being swivelled about an axis extending almost vertically in relation to the strip.
In all of these conventional or proposed types of embodiment, the stripwisely divided flat snapping spring plate is shortened by way of bendings on at least one strip, and is thus distorted. Common practice has shown, however, that it is very difficult to produce snapping spring plates having reproducible properties, and in particular, that it is difficult to produce snapping spring plates for smaller types of switches in such a way that all of the manufactured snapping spring plates have the same snap-over characteristic. For manufacturing such snapping spring plates it is necessary to use a hard spring material having very specific spring or elastic qualities. For effecting the shortening, however, it is necessary to bend the sheet metal over a sharp bending edge and to employ a considerable overbending force in order to obtain the desired remaining deformation. This deformation, however, is strongly dependent upon the material itself, of its thickness and its hardness. Moreover, the acute-angled prebending substantially reduces the service life of the flat spring. If, on the other hand, deformations having a large bending radius are used, which is anyway only possible in the case of larger snapping spring plates, the sheet metal material is not stressed to the same extent when producing the deformation, but it has proved that the thus produced deformation does not remain constant, and in fact varies in the course of time. This, however also changes the snapping properties of the sheet metal material, which are dpendent upon the size of the shortening and, consequently of the distortion of the sheet metal spring. Obviously, when producing the shortening, the internal structure of the sheet metal is substantially changed at the bending point, with this having a substantial influence upon the mechanical stability and the elastic properties of the sheet metal material. This may well be one of the reasons for the fact that it is extremely difficult with manufactured snapping spring plates, to produce the required shortenings in such a way that the thus obtained flat snapping spring plates all have the same snapping properties. As already denoted hereinbefore, there is still to be added that the change in the properties of the material caused by the produced bending, is still dependent on slight thickness variations in the sheet metal, as well as upon deviations regarding the hardness of the individual sheet metals. It has proved that even when using sheet metals showing to have only very slight deviations in their thickness, and very uniform elastic properties, the flat snapping spring plates which are obtained after the deformation has been effected, still have considerable deviations with regard to the snapping properties. This, however, is noticed very disturbingly in cases where a keyboard is constituted of a large number of such snap-action switches, with the snapping properties of the individual flat snapping spring plates still being subject to variations or changes during the time they are used. In this respect it still has to be considered that in the case of relatively small snap-action switches, the actuating paths are only very small.
It is the object of the present invention to provide snap-action switches of the type mentioned hereinbefore, avoiding the difficulties in manufacturing the formed-in shortening, and which have reproducible and uniform snapping properties.